The present invention relates to an amplifying solid-state imaging device and a method for driving the same.
An amplifying solid-state imaging device, as well as a CCD solid-state imaging device, adopts “electronic shuttering” as a sort of electronic diaphragm. The “electronic shuttering” operation is performed to reset a signal charge storage section just before a photodiode in each pixel starts to store the signal charge, which has been created by the photodiode itself through photoelectric conversion, thereby making the charge storage period of the photodiode variable. The signal charge stored in pixels is read out on a row-by-row basis responsive to a horizontal sync signal. Thus, the electronic shuttering operation is also performed on the row-by-row basis (which is called a “focal plane operation”). More specifically, the electronic shuttering operation is performed on a certain row and then the signal charge starts to be stored. And after a predetermined time has passed since the start of charge storage, a signal readout operation is performed. When the readout operation is started, the storage section is reset again for the readout. The “predetermined time” defines the charge storage period of each photodiode and is of an equal length for every row. Accordingly, supposing each pixel is receiving light with the same intensity, the same quantity of charge will be stored on each and every row theoretically speaking.
FIG. 1 illustrates a schematic configuration of a conventional amplifying solid-state imaging device 100. In the device 100, an imaging section is made up of a plurality of pixels 102 arranged in columns and rows. Each of these pixels 102 includes a photodiode for storing charge in a quantity corresponding to the amount of light received. As shown in FIG. 1, a row select encoder 103 for selecting one pixel row after another from the imaging section is disposed on the right-hand side of the imaging section. In the example illustrated in FIG. 1, the number of pixel rows is m, which is equal to or larger than two. The row select encoder 103 includes the number m of row selectors that are connected in series to each other. An ith (where 1≦i≦m) row selector generates a reset signal for the electronic shuttering operation at a predetermined time, and then sends the signal to all the pixels 102 belonging to the ith row. The row selectors included in the row select encoder 103 output the reset signal for the electronic shuttering operation at respectively times, which are different from each other among the rows. That is to say, the reset signal is sequentially output in the descending order, i.e., from the first through mth rows.
On the other hand, the row selecting operation for readout (i.e., an ordinary row selection) is also performed sequentially by the number m of row selectors. The interval between the electronic shuttering and readout operations is preset at the same length for every row. The signal read out from a selected row is supplied by a column select driver 107 to an output buffer 111, from which the signal is output as a pixel signal.
When the electronic shuttering operation is performed, the photodiode within each pixel 102 has the potential thereof compulsorily reset at a predetermined level (which will be called a “reset potential” in this specification). The reset potential is supposed to be the same in each and every pixel 102 as a matter of principle. However, the present inventors found that since a reset potential on a certain row might shift into a different reset potential on another row, horizontal noise might appear on the screen as a result. The horizontal noise always appears on a particular set of rows on the screen, thus degrading the resultant image quality.